Today there is intense interest in thin glass with precision formed holes for electronics applications. The holes are filled with a conducting material, and are used to conduct electrical signals from one part to another, allowing precision connection of central processing units, memory chips, graphical processing units, or other electronic components. For such applications, substrates with metalized holes in them are typically called “interposers.” As compared to presently used interposer materials such as fiber reinforced polymer or silicon, glass has a number of advantageous properties. Glass can be formed thin and smooth in large sheets without the need for polishing, it has higher stiffness and greater dimensional stability than organic alternatives, it is a much better electrical insulator than silicon, it has better dimensional (thermal and rigidity) stability than organic options, and it can be tailored to different coefficients of thermal expansion to control stack warp in integrated circuits.
A variety of hole formation methods can be used to create holes in glass, such as hot pressing, lithography of photo-machinable glass, electric-discharge drilling, powder blasting, and a wide variety of laser drilling methods. With any of the techniques, the challenge is generally around forming a hole of sufficient quality (low cracking, appropriate size or roundness) at a high enough rate (holes/sec) which ultimately affects cost. For example, hot pressing of glass has difficulty forming holes of small enough dimensions (less than or equal to about 100 microns), electrical discharge drilling can be difficult to do with a tight hole pitch (i.e., a hole to hole distance of less than about 50 microns), laser drilling of holes using beam trepanning can be slow (e.g., about 1 hole/sec), and excimer laser processing and photomachinable glass can have large initial capital costs.
Laser drilling methods with UV nanosecond lasers have been demonstrated that make particularly high quality holes. A laser is used to make about 10 micron diameter pilot holes using multiple (e.g., hundreds) of laser pulses per hole, and then the part is etched with acid to enlarge the holes and achieve the target dimensions. The holes are subsequently metalized, redistribution layers are added to fan out electrical signals, and the parts are diced into smaller pieces to create functional interposers. However, laser drilling can be a time consuming process, and with percussion drilling (i.e., one pulse after another at the same location), it can take hundreds of pulses to drill an individual hole to the desired depth. As the capital cost of a precision laser drilling platform can be significant (approaching $1M/machine), speed of hole formation is a key parameter in overall interposer production cost.
Therefore, there is a need for a method of laser drilling a material, such as glass, that minimizes or eliminates the above mentioned problems.